Through the official Blizzard website, the site has revealed that that every loot box always contains at least one Rare (blue) item. On average, players receive an Epic (purple) item every 5.5 loot boxes and a Legendary (orange) item every 13.5 loot boxes

The numbers apply for the Chinese version of the game of course, but these numbers most likely apply to every version of the game. Overwatch is available to purchase on PC, PS4, and Xbox One